Liquid crystal display device with thick interlayer insulating film under pixel electrode

ABSTRACT

The transmission type liquid crystal display device of this invention includes: gate lines; source lines; and switching elements each arranged near a crossing of each gate line and each source line, a gate electrode of each switching element being connected to the gate line, a source electrode of the switching element being connected to the source line, and a drain electrode of the switching element being connected to a pixel electrode for applying a voltage to a liquid crystal layer, wherein an interlayer insulating film formed of a fluoric resin with high transparency is provided above the switching element, the gate line, and the source line, and wherein the pixel electrode formed of a transparent conductive film is provided on the interlayer insulating film.

RELATED APPLICATION

This application is related copending commonly asigned application Ser.No. 08/695,632 filed Aug. 12, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission type liquid crystaldisplay device which includes switching elements such as thin filmtransistors (hereinafter, referred to as “TFTs”) as addressing elementsand is used for displays of computers, TV sets, and the like, and amethod for fabricating such a transmission type liquid crystal displaydevice.

2. Description of the Related Art

FIG. 16 is a circuit diagram of a conventional transmission type liquidcrystal display device provided with an active matrix substrate.

Referring to FIG. 16, the active matrix substrate includes a pluralityof pixel electrodes 1 arranged in a matrix and TFTs 2 used as switchingelements connected to the respective pixel electrodes 1. Gate electrodesof the TFTs 2 are connected to gate lines 3 for supplying a scanning(gate) signal, so that the gate signal can be input into the gateelectrodes to control the driving of the TFTs 2. Source electrodes ofthe TFTs 2 are connected to source lines 4 for supplying an image (data)signal, so that the data signal can be input into the correspondingpixel electrodes 1 via the TFTs 2 when the TFTs 2 are being driven. Thegate lines 3 and the source lines 4 run adjacent to the pixel electrodes1 and are arranged in a matrix to cross each other. Drain electrodes ofthe TFTs 2 are connected to the respective pixel electrodes 1 andstorage capacitors 5. Counter electrodes of the storage capacitors 5 areconnected to common lines 6. The storage capacitor 5 is used for holdinga voltage applied to a liquid crystal layer. The storage capacitor isprovided in parallel to a liquid crystal capacitor which includes theliquid crystal layer sandwiched between a pixel electrode provided on anactive matrix substrate and a counter electrode provided on a countersubstrate.

FIG. 17 is a sectional view of a one-TFT portion of the active matrixsubstrate of the conventional liquid crystal display device.

Referring to FIG. 17, a gate electrode 12 connected to the gate line 3shown in FIG. 16 is formed on a transparent insulating substrate 11. Agate insulating film 13 is formed covering the gate electrode 12. Asemiconductor layer 14 is formed on the gate insulating film 13 so as tooverlap the gate electrode 12 via the gate insulating film 13, and achannel protection layer 15 is formed on the center of the semiconductorlayer 14. n⁺-Si layers as a source electrode 16 a and a drain electrode16 b are formed covering the end portions of the channel protectionlayer 15 and portions of the semiconductor layer 14, so that they areseparated from each other at the top of the channel protection layer 15.A metal layer 17 a which is to be the source line 4 shown in FIG. 16 isformed to overlap the source electrode 16 a as one of the n⁺-Si layers.A metal layer 17 b is formed to overlap the drain electrode 16 b as theother n⁺-Si layer so as to connect the drain electrode 16 b and thepixel electrode 1. An interlayer insulating film 18 is formed coveringthe TFT 2, the gate line 3, and the source line 4.

A transparent conductive film is formed on the interlayer insulatingfilm 18 to constitute the pixel electrode 1. The transparent conductivefilm is connected to the metal layer 17 b which is in contact with thedrain electrode 16 b of the TFT 2 via a contact hole 19 formed throughthe interlayer insulating film 18.

Thus, since the interlayer insulating film 18 is formed between thepixel electrode 1 and the underlying layers including the gate andsource lines 3 and 4, it is possible for the pixel electrode 1 tooverlap the lines 3 and 4. Such a structure is disclosed in JapaneseLaid-Open Patent Publication No. 58-172685, for example. With thisstructure, the aperture ratio improves and, since the electric fieldgenerated by the lines 3 and 4 is shielded, the occurrence ofdisclination can be minimized.

Conventionally, the interlayer insulating film 18 is formed bydepositing an inorganic material such as silicon nitride (SiN) to athickness of about 500 nm by chemical vapor deposition (CVD).

The above conventional liquid crystal display device has disadvantagesas follows.

When a transparent insulating film made of SiN_(x), SiO₂, TaO_(x), andthe like is formed on the interlayer insulating film 18 by CVD orsputtering, the surface of the film directly reflects the surfaceprofile of the underlying film, i.e., the interlayer insulating film 18.Therefore, when the pixel electrode 1 is formed on the transparentinsulating film, steps of 100 to 200 nm, in general, will be formed onthe pixel electrode 1 if the underlying film has steps, causingdisturbance in the orientation of liquid crystal molecules.

The other disadvantage is as follows. When the pixel electrode 1overlaps the gate line 3 and the source line 4 via the interlayerinsulating film 18, the capacitances between the pixel electrode 1 andthe gate line 3 and between the pixel electrode 1 and the source line 4increase. In particular, when an inorganic film made of silicon nitrideand the like is used as the interlayer insulating film 18, thedielectric constant of such a material is as high as 8 and, since thefilm is formed by CVD, the thickness of the resultant film is as smallas about 500 nm. With such a thin interlayer insulating film, thecapacitances between the pixel electrode 1 and the lines 3 and 4 arelarge. This causes the following problems (1) and (2). Incidentally, inorder to obtain a thicker inorganic film made of silicon nitride and thelike, an undesirably long time is required in the aspect of thefabrication process.

(1) When the pixel electrode 1 overlaps the source line 4, thecapacitance between the pixel electrode 1 and the source line 4 becomeslarge. This increases the signal transmittance, and thus a data signalheld in the pixel electrode 1 during a holding period fluctuatesdepending on the potential thereof. As a result, the effective voltageapplied to the liquid crystal in the pixel varies, causing, inparticular, vertical crosstalk toward a pixel adjacent in the verticaldirection in the actual display.

In order to reduce the influence of the capacitance between the pixelelectrode 1 and the source line 4 appearing on the display, JapaneseLaid-Open Patent Publication No. 6-230422 proposes a driving methodwhere the polarity of a data signal to be supplied to the pixels isinverted every source line. This driving method is effective for ablack-and-white display panel where the displays (i.e., data signals) ofadjacent pixels are highly correlated with each other. However, it isnot effective for a color display panel for normal notebook typepersonal computers and the like where pixel electrodes are arranged in avertical stripe shape (in color display, a square pixel is divided intothree vertically long rectangular picture elements representing R, G,and B, forming a vertical stripe shape). The display color of pixelsconnected to one source line is different from that of pixels connectedto an adjacent source line. Accordingly, the proposed driving method ofinverting the polarity of the data signal every source line is noteffective in reducing crosstalk for the general color display, though itis effective for the black-and-white display.

(2) When the pixel electrode 1 overlaps the gate line 3 for driving thepixel, the capacitance between the pixel electrode 1 and the gate line 3becomes large, increasing the feedthrough of the write voltage to thepixel due to a switching signal for controlling the TFT 2.

SUMMARY OF THE INVENTION

A transmission type liquid crystal display device of this inventionincludes: gate lines; source lines; and switching elements each arrangednear a crossing of each gate line and each source line. A gate electrodeof each switching element is connected to the gate line, a sourceelectrode of the switching element is connected to the source line, anda drain electrode of the switching element is connected to a pixelelectrode for applying a voltage to a liquid crystal layer, wherein aninterlayer insulating film formed of a fluoric resin with hightransparency is provided above the switching element, the gate line, andthe source line. The pixel electrode formed of a transparent conductivefilm is provided on the interlayer insulating film.

In one embodiment of the invention, the device further includes aconnecting electrode for connecting the pixel electrode and the drainelectrode, wherein the interlayer insulating film is provided above theswitching element, the gate line, the source line, and the connectingelectrode. The pixel electrode is formed on the interlayer insulatingfilm so as to overlap at least the gate line or the source line at leastpartially, and the connecting electrode and the pixel electrode areconnected with each other via a contact hole formed through theinterlayer insulating film.

In one embodiment of the invention, the pixel electrode and at least oneof the source line and the gate line overlap each other by about 1 μm ormore in a line width direction.

In one embodiment of the invention, the thickness of the interlayerinsulating film is about 1.2 μm or more.

In one embodiment of the invention, the connecting electrode is formedof a transparent conductive film.

In one embodiment of the invention, the device further includes astorage capacitor for maintaining a voltage applied to the liquidcrystal layer, wherein the contact hole is formed above either anelectrode of the storage capacitor or the gate line.

In one embodiment of the invention, a metal nitride layer is formedbelow the contact hole to connect the connecting electrode and the pixelelectrode.

In one embodiment of the invention, the device further includes astorage capacitor for holding a voltage applied to the liquid crystallayer, wherein a capacitance ratio represented by expression (1):

Capacitance ratio=C _(sd)/(C _(sd) +C _(ls) +C _(s))  (1)

is 10% or less, wherein C_(ad) denotes a capacitance value between thepixel electrode and the source line, C_(ls) denotes a capacitance valueof a liquid crystal portion corresponding to each pixel in anintermediate display state, and C_(s) denotes a capacitance value of thestorage capacitor of each pixel.

In one embodiment of the invention, the shape of the pixel electrode isrectangular with a side parallel to the gate line being longer than aside parallel to the source line.

In one embodiment of the invention, the device further includes adriving circuit for supplying to the source line a data signal of whichpolarity is inverted for every horizontal scanning period, and the datasignal is supplied to the pixel electrode via the switching element. Thedata signal having a phase difference when compared with a gate signal.

In one embodiment of the invention, the device further includes astorage capacitor for holding a voltage applied to the liquid crystallayer. The storage capacitor includes a storage capacitor electrode, astorage capacitor counter electrode and an insulating film therebetween,wherein the storage capacitor electrode is formed in the same layer aseither the source line or the connecting electrode.

In one embodiment of the invention, the storage capacitor counterelectrode is formed of a part of the gate line.

In one embodiment of the invention, the pixel electrode and the storagecapacitor electrode are connected via the contact hole formed above thestorage capacitor electrode.

In one embodiment of the invention, the contact hole is formed aboveeither the storage capacitor counter electrode or the gate line.

According to another aspect of the invention, a method for fabricating atransmission type liquid crystal display device is provided. The methodincludes the steps of: forming a plurality of switching elements in amatrix on a substrate; forming a gate line connected to a gate electrodeof each switching element and a source line connected to a sourceelectrode of the switching element, the gate line and the source linecrossing each other; forming a connecting electrode formed of atransparent conductive film connected to a source electrode of theswitching element; forming a fluoric film with high transparency abovethe switching elements, the gate lines, the source lines, and theconnecting lines by a coating method and patterning the fluoric film toform an interlayer insulating film and contact holes through theinterlayer insulating film to reach the connecting electrodes; andforming pixel electrodes formed of transparent conductive films on theinterlayer insulating film and inside the contact holes so that eachpixel electrode overlaps at least either the gate line or the sourceline at least partially.

In one embodiment of the invention, the patterning of the fluoric filmis conducted by the step of etching the fluoric film by using aphotoresist on the fluoric film as an etching mask.

In one embodiment of the invention, the patterning of the fluoric filmincludes the steps of: forming a photoresist layer containing silicon onthe fluoric film; patterning the photoresist layer; and etching thefluoric film by using the patterned photoresist layer as an etchingmask.

In one embodiment of the invention, the patterning of the fluoric filmincludes the steps of: forming a photoresist layer on the fluoric film;coating a silane coupling agent on the photoresist layer and oxidizingthe coupling agent; patterning the photoresist layer; and etching thefluoric film by using the patterned photoresist layer covered with theoxidized coupling agent as an etching mask.

In one embodiment of the invention, the etching step is a step of dryetching using an etching gas containing at least one of CF₄, CF₃H andSF₆.

In one embodiment of the invention, the fluoric resin for forming theinterlayer insulating film has a light transmittance of 90% or more forlight with a wavelength in the range of about 400 nm to about 800 nm.

In one embodiment of the invention, the fluoric film has a thickness ofabout 1.2 μm or more.

In one embodiment of the invention, the method further includes the stepof, before the formation of the fluoric film, irradiating withultraviolet light a surface of the substrate where the fluoric film isto be formed.

In one embodiment of the invention, the method further includes the stepof, before the formation of the fluoric film, applying a silane couplingagent on a surface of the substrate where the fluoric film is to beformed.

In one embodiment of the invention, the material for forming the fluoricfilm contains a silane coupling agent.

In one embodiment of the invention, the silane coupling agent includesat least one of hexamethyl disilazane, dimethyl diethoxy silane, andn-buthyl trimethoxy.

In one embodiment of the invention, the method further includes the stepof, before the formation of the pixel electrode, ashing the surface ofthe interlayer insulating film by an oxygen plasma.

In one embodiment of the invention, the ashing step is conducted afterthe formation of the contact holes.

In one embodiment of the invention, the interlayer insulating filmincludes a thermally curable material and the interlayer insulating filmis cured before the ashing step.

In one embodiment of the invention, the thickness of the ashed portionof the interlayer insulating film is in the range of about 100 to 500nm.

In one embodiment of the invention, the thickness of the pixel electrodeis about 50 nm or more.

In one embodiment of the invention, the method further includes the stepof, before the formation of the fluoric film, forming a silicon nitridefilm on a surface of the substrate where the fluoric film is to beformed.

Thus, the invention described herein makes possible the advantage of (1)providing a transmission type liquid crystal display device where flatpixel electrodes overlap respective lines to improve the aperture ratioof the liquid crystal display, minimize disturbance in the orientationof liquid crystal molecules, and simplify the fabrication process.Furthermore, the influence of the capacitance between the pixelelectrodes and the lines appearing on the display, such as crosstalk,can be reduced to achieve a good display. The invention described hereinalso makes possible the advantage of (2) providing a method forfabricating such a transmission type liquid crystal display device.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 1 according to the present invention.

FIG. 2 is a sectional view taken along line A-A′ of FIG. 1.

FIG. 3 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 2 according to the present invention.

FIG. 4 is a sectional view taken along line B-B′ of FIG. 3.

FIG. 5 is a partial sectional view of an active matrix substrate of atransmission type liquid crystal display device of Example 3 accordingto the present invention.

FIG. 6 is a partial sectional view of an active matrix substrate ofExample 4 according to the present invention.

FIG. 7 is a graph illustrating the relationship between the liquidcrystal charging rate difference and the capacitance ratio fortransmission type liquid crystal display devices of Examples 5 and 6 anda conventional liquid crystal display device.

FIGS. 8A and 8B, re waveforms of data signals in the cases of 1Hinversion driving in Examples 5 and 6 and conventional field inversiondriving, respectively.

FIG. 9 is a graph illustrating the relationship between the liquidcrystal capacitance ratio and the overlap width for the transmissiontype liquid crystal display device of Example 5.

FIG. 10 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 7 according to the present invention.

FIG. 11 is a sectional view taken along line C-C′ of FIG. 10.

FIG. 12 is a circuit diagram of a C_(s)-on-gate type liquid crystaldisplay device.

FIG. 13 is a plan view of a one-pixel portion of an active matrixsubstrate obtained by applying the structure of Example 2 to the liquidcrystal display device shown in FIG. 12.

FIG. 14 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 10 according to the present invention.

FIG. 15 is a sectional view taken along line D-D′ of FIG. 14.

FIG. 16 is a circuit diagram of a conventional liquid crystal displaydevice provided with an active matrix substrate.

FIG. 17 is a sectional view of a one-pixel portion of the active matrixsubstrate of the conventional liquid crystal display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by way of examples withreference to the accompanying drawings.

EXAMPLE 1

FIG. 1 is a plan view of a one-pixel portion of an active matrixsubstrate of the transmission type liquid crystal display device ofExample 1 according to the present invention.

Referring to FIG. 1, the active matrix substrate includes a plurality ofpixel electrodes 21 (within a bold line) arranged in a matrix. Gatelines 22 for supplying a scanning (gate) signal and source lines 23 forsupplying an image (data) signal run surround the peripheries of thepixel electrodes 21 and cross each other. The peripheries of each pixelelectrode 21 overlap the gate lines 22 and the source lines 23. A TFT 24acting as a switching element connected to the corresponding pixelelectrode 21 is formed at a crossing of the gate line 22 and the sourceline 23. A gate electrode of the TFT 24 is connected to the gate line 22so that a gate signal can be input into the gate electrode to controlthe driving of the TFT 24. A source electrode of the TFT 24 is connectedto the source line 23 so that a data signal can be input into the sourceelectrode. A drain electrode of the TFT 24 is connected to the pixelelectrode 21 via a connecting electrode 25 and a contact hole 26. Thedrain electrode is also connected to an electrode of a storage capacitor(a storage capacitor electrode 25 a) via the connecting electrode 25.The other electrode of the storage capacitor, a storage capacitorcounter electrode 27, is connected to a common line (element 6 in FIG.16).

FIG. 2 is a sectional view of the active matrix substrate taken alongline A-A′ of FIG. 1.

Referring to FIG. 2, a gate electrode 32 connected to the gate line 22shown in FIG. 1 is formed on a transparent insulating substrate 31. Agate insulating film 33 is formed covering the gate electrode 32. Asemiconductor layer 34 is formed on the gate insulating film 33 so as tooverlap the gate electrode 32 via the gate insulating film 33, and achannel protection layer 35 is formed on the center of the semiconductorlayer 34. n⁺-Si layers as a source electrode 36 a and a drain electrode36 b are formed covering the end portions of the channel protectionlayer 35 and portions of the semiconductor layer 34, so that they areseparated from each other by a portion of the channel protection layer35. A transparent conductive film 37 a and a metal layer 37 b which areto be the double-layer source line 23 shown in FIG. 1 are formed tooverlap the source electrode 36 a as one of the n⁺-Si layers. Atransparent conductive film 37 a′ and a metal layer 37 b′ are formed tooverlap the drain electrode 36 b as the other n⁺-Si layer. Thetransparent conductive film 37 a′ extends to connect the drain electrode36 b and the pixel electrode 21 and also serves as the connectingelectrode 25 which is connected to the storage capacitor electrode 25 aof the storage capacitor. An interlayer insulating film 38 is formedcovering the TFT 24, the gate line 22, the source line 23, and theconnecting electrode 25.

A transparent conductive film is formed on the interlayer insulatingfilm 38 to constitute the pixel electrode 21. The pixel electrode 21 isconnected to the drain electrode 36 b of the TFT 24 via the contact hole26 formed through the interlayer insulating film 38 and the transparentconductive film 37 a′ which is the connecting electrode 25. In order tomaintain an excellent brightness characteristic of the display device,the interlayer insulating film 38 is preferably made of a materialhaving a light transmittance of 90% or more for light with a wavelengthin the range of about 400 nm to 800 nm.

Examples of fluoric resin include fluoric polyimides obtained by usingacid anhydrides including a fluorine atom or diamines including afluorine atom with acid anhydrides. In addition, anhydrides such asoxydiphthalic acid anhydride, and biphenyl tetracaboxylic acid anhydrideor meta-substituted aromatic diamines having a sulfone group and/or anether group can be used with the fluorine-containing acid anhydrides andthe fluorine-containing diamines. These polyimide resins are disclosedin Fujita, et al., Nitto Giho, Vol. 29, No. 1, pp. 20-28 (1991), forexample. Among the above transparent and colorless fluoric polyimideresins, a resin containing both acid anhydride and diamine each having ahexafluoropropylene group has a high transparency. Fluoric resins otherthan the above fluoric polyimides can also be used, such as fluoricacrylate resins, which is a photosensitive resin. Fluoric materials havenot only excellent colorless transparency but also a low dielectricconstant and high heat resistance.

Specifically, the above fluoric polyimides are synthesized, for example,by any one of the following formulae (1) through (3),

The active matrix substrate with the above structure is fabricated asfollows.

First, the gate electrode 32, the gate insulating film 33, thesemiconductor layer 34, the channel protection layer 35, and the n⁺-Silayers as the source electrode 36 a and the drain electrode 36 b aresequentially formed in this order on the transparent insulatingsubstrate 31 such as a glass substrate. This film formation step can beconducted following a conventional method for fabricating an activematrix substrate.

Thereafter, the transparent conductive films 37 a and 37 a′ and themetal layers 37 b and 37 b′ constituting the source line 23 and theconnecting electrode 25 are sequentially formed by sputtering and arepatterned into a predetermined shape.

A fluoric resin is applied to the resultant substrate, for example, byspin coating and cured to a thickness of 2 μm, to form the interlayerinsulating film 38. The resultant resin layer is shielded with a resistand exposed to light according to a predetermined pattern. A portion ofthe resist is removed, for example, with an alkaline solution and dryetching is conducted using the resist as a mask to form the contactholes 26 through the interlayer insulating film 38. Thereafter, theremaining resist is removed. Dry etching allows an accurate and smoothcontact hole 26 to be formed and thus the occurrence of disconnection ofthe pixel electrode 21 is reduced.

Subsequently, a transparent conductive film is formed on the resultantsubstrate by sputtering and is patterned to form the pixel electrodes21. Each pixel electrode 21 is thus connected to the correspondingtransparent conductive film 37 a′ which is in contact with the drainelectrode 36 b of the TFT 24 via the contact hole 26 formed through theinterlayer insulating film 38. In this way, the active matrix substrateof this example is fabricated.

The thus-fabricated active matrix substrate includes the thickinterlayer insulating film 38 between the pixel electrode 21 and theunderlying layers including the gate line 22, the source line 23, andthe TFT 24. With this thick interlayer insulating film, it is possiblefor the pixel electrode 21 to overlap the gate and source lines 22 and23 and the TFT 24. Also, the surface of the pixel electrode 21 can bemade flat. As a result, when the transmission type liquid crystaldisplay device including the thus-fabricated active matrix substrate anda counter substrate with a liquid crystal layer therebetween iscompleted, the aperture ratio of this device can be improved. Also,since the electric field generated at the lines 22 and 23 can beshielded, the occurrence of disclination can be minimized.

The fluoric resin constituting the interlayer insulating film 38 has adielectric constant of 2 to 3 which is lower than that of an inorganicfilm (e.g., the dielectric constant of silicon nitride is 8) and a hightransparency. Also, since the spin coating is employed, a thickness aslarge as 2 μm can be easily obtained. This reduces the capacitancesbetween the gate line 22 and the pixel electrode 21 and between thesource lines 23 and the pixel electrodes 21, lowering the time constant(signal transmittance). As a result, the influence of the capacitancesbetween the lines 22 and 23 and the pixel electrode 21 appearing on thedisplay, such as crosstalk, can be reduced, and thus a good and brightdisplay can be obtained. The contact hole 26 can be formed into a sharptapered shape by the patterning including the exposure to light and thealkaline development. This facilitates a better connection between thepixel electrode 21 and the transparent conductive film 37 a′.

Since the thickness of the interlayer insulating film 38 is as large asseveral micrometers, thicker than that in conventional liquid crystaldisplay, a resin with a transmittance as high as possible is preferablyused. The visual sensitivity of a human eye for blue is a little lowerthan those for green and red. Accordingly, even if the spectraltransmittance of the film has slightly lower transmittance for bluelight than that for green and red light, the display quality will be notsubstantially deteriorated. Though the thickness of the interlayerinsulating film 38 was made 2 μm in this example, it is not limited to 2μm. The thickness of the interlayer insulating film may be set dependingon the transmittance and the dielectric constant of the film. In orderto reduce the capacitance, the thickness is preferably equal to orgreater than about 1.2 μm, more preferably equal to or greater thanabout 1.5 μm.

Since a fluorine containing material has a lower dielectric constantthan that of a general organic material, it is possible to form thinnerfilms. As a result, smaller contact holes can be obtained lowering theproduction cost. Furthermore, for the same reason, when an aging test(e.g. at 80° C.) is conducted, for example, the degree of polarizationinduced in the interlayer insulating film by the electric field betweenthe TFT 24 and the pixel electrode 21 is relatively small. As a result,there is little fluctuation in TFT characteristics and thus, highreliability thereof can be obtained. Moreover, since fluoric resins haveexcellent chemical resistance, a wider variety of chemicals can be usedin the later steps which simplifies the fabrication process and reducesthe production cost.

In this example, the transparent conductive film 37 a′ is formed as theconnecting electrode 25 connecting the drain electrode 36 b of each TFT24 and the corresponding pixel electrode 21. This is advantageous in thefollowing points. In the conventional active matrix substrate, theconnecting electrode is composed of a metal layer. When such a metalconnecting electrode is formed in the aperture portion, the apertureratio is lowered. In order to overcome this problem, the connectingelectrode is conventionally formed above the TFT or the drain electrodeof the TFT. The contact hole is formed above the connecting electrodethrough the interlayer insulating film to connect the drain electrode ofthe TFT and the pixel electrode. With this conventional structure,however, when the TFT is made smaller to improve the aperture ratio, forexample, it is not possible to accommodate the entire contact hole abovethe smaller TFT. As a result, the aperture ratio is not improved. Whenthe thickness of the interlayer insulating film is made as large asseveral micrometers, the contact hole should be tapered in order toconnect the pixel electrode and the underlying connecting electrode, anda large-size connecting electrode is required in the TFT region. Forexample, when the diameter of the contact hole is 5 μm, the size of theconnecting electrode should be about 14 μm in consideration of thetapered contact hole and the alignment allowance (the thickness of theinterlayer insulating film can be made thinner than that of aninterlayer insulating film made of other organic materials and thus thesize thereof can be smaller). In the conventional active matrixsubstrate, if a TFT with a size smaller than this value is realized, theoversized connecting electrode causes a new problem of lowering theaperture ratio. In contrast, in the active matrix substrate of thisexample, since the connecting electrode 25 is composed of thetransparent conductive film 37 a′, no trouble of lowering the apertureratio arises. Further, in this example, the connecting electrode 25extends to connect the drain electrode 36 b of the TFT and the storagecapacitor electrode 25 a of the storage capacitor formed by thetransparent conductive film 37 a′. Since the extension is also formed ofthe transparent conductive film 37 a′, it does not lower the apertureratio, either.

In this example, the source line 23 is of a double-layer structurecomposed of the transparent conductive layer 37 a and the metal layer 37b. If part of the metal layer 37 b is defective, the source line 23 canremain electrically conductive through the transparent conductive film37 a, so that the occurrence of disconnection of the source line 23 canbe reduced.

A photoresist containing silicon is preferably used as the photoresistfor the patterning of the interlayer insulating film made of a fluoricresin. In the above etching, dry etching is normally conducted using agas containing CF₄, CF₃H, SF₆ and the like. In this case, however, sincethe photoresist and the interlayer insulating film are both organicresins, it is difficult to increase the selection ratio between theseresins. This is especially true in the case where the thickness of theinterlayer insulating film is as large as 1.2 μm or more which is nearlythe same as that of the photoresist, as in this example. It ispreferable that the materials have sufficiently different etching rates(i.e., selectivity). When a fluoric resin is used as the interlayerinsulating film in combination with a common photoresist material (e.g.,OFPR-800 produced by Tokyo Ohka Kogyo Co., Ltd.) is used, for example,the selection ratio is about 1.5. In contrast, in this example, by usingthe photoresist containing silicon, a selectivity with respect to thefluoric resin of about 2.0 or more can be obtained. Therefore,patterning with high precision is attained.

Alternatively, at the formation of the interlayer insulating film by thepatterning using a photoresist which does not contain silicon, a silanecoupling agent (e.g., hexamethyl disilazane) may be applied to thephotoresist and the silane coupling agent layer is treated with oxygenplasma to form a silicon oxide film. As a result, the etching rate ofthe photoresist is reduced, since the silicon oxide film on thephotoresist serves as a protection film. This method can be used incombination with the silicon containing photoresist.

The increase in the selection ratio by the above mentioned methodutilizing a silicon element is especially effective in the dry etchingusing a gas containing CF₄, CF₃H or SF₆.

The active matrix substrate with the thus-formed interlayer insulatingfilm 38 can provide a high aperture ratio.

EXAMPLE 2

In Example 2, another method for forming the interlayer insulating film38 will be described.

FIG. 3 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 2 according to the present invention. FIG. 4 is a sectional viewtaken along line B-B′ of FIG. 3. Components having like functions andeffects are denoted by the same reference numerals as those in FIGS. 1and 2, and the description thereof is omitted.

In the active matrix substrate of this example, each contact hole 26 ais formed above the storage capacitor electrode 25 a and the storagecapacitor counter electrode 27 of the storage capacitor of each pixel.As described in Example 1, the storage capacitor electrode 25 aconstitutes the end portion of the connecting electrode 25 which isconnected to the drain electrode 36 b of the TFT 24. The other electrodeof the storage capacitor, the storage capacitor counter electrode 27, isconnected to a counter electrode formed on a counter substrate via thestorage capacitor common line 6 shown in FIG. 16. In other words, thecontact holes 26 a are formed above the storage capacitor common line 6which is composed of a light-shading metal film.

The above structure of the active matrix substrate of this example hasthe following advantages.

Since the thickness of the interlayer insulating film 38 is as large as2 μm, for example, which is well comparable with the thickness of aliquid crystal cell of 4.5 μm, light leakage tends to occur around thecontact holes 26 a due to a disturbance in the orientation of the liquidcrystal molecules. If the contact holes 26 a are formed in the apertureportions of the transmission type liquid crystal display device, thecontrast is lowered due to the light leakage. On the other hand, theactive matrix substrate of this example is free from this troublebecause each contact hole 26 a is formed above the storage capacitorelectrode 25 a and the storage capacitor counter electrode 27 as an endportion of the storage capacitor common line 6 composed of alight-shading metal film. In other words, as long as the contact hole 26a is formed above the storage capacitor common line 6 composed of alight-shading metal film, not in the aperture portion, any light leakagewhich may occur around the contact hole 26 a due to a disturbance in theorientation of the liquid crystal molecules will not result in loweringof the contrast. This also applies to the case where the storagecapacitor is formed using a portion of the adjacent gate line 22 as oneof electrodes thereof. In this case, the contact hole 26 a is formedabove the light-shading gate line 22 and thus lowering of the contrastcan be prevented.

In the active matrix substrate of this example, the connecting electrode25 for connecting the drain electrode 36 b of the TFT 24 and the contacthole 26 a is composed of the transparent conductive film 37 a′.Accordingly, the aperture ratio does not become lower by forming thecontact hole 26 a above the storage capacitor. The size of the contacthole 26 a is smaller in the case where a fluoric interlayer insulatingfilm is used as the interlayer insulating film than in the case whereanother material is used. The smaller size of the contact hole allowsfreer alignment of the contact hole with the storage capacitor.

If the storage capacitor counter electrode 27 formed under the contacthole 26 a is formed to shade light, light leakage which may occur due toa disturbance in the orientation of the liquid crystal molecules doesnot influence the display. The size of the contact hole 26 a is notnecessarily so precise, allowing the hole to be larger and smoother. Asa result, the pixel electrode 21 formed on the interlayer insulatingfilm 38 is continuous, not being interrupted by the contact hole 26 a.This improves the production yield.

EXAMPLE 3

FIG. 5 is a partial sectional view of an active matrix substrate of thetransmission type liquid crystal display device of Example 3 accordingto the present invention. Components having like functions and effectsare denoted by the same reference numerals as those in FIGS. 1 to 4, andthe description thereof is omitted.

In the active matrix substrate of this example, each contact hole 26 bis formed through the interlayer insulating film 38 above the storagecapacitor common line 6. A metal nitride layer 41 is formed on theportion of the transparent conductive film 37 a′ under each contact hole26 b.

The above structure of the active matrix substrate of this example isadvantageous in the following point.

Some troubles arise in the adhesion between the resin used for theinterlayer insulating film 38 and ITO (indium tin oxide) used for thetransparent conductive film or metal such as Ta and Al. For example, inthe cleaning process after the formation of the contact hole 26 b, acleaning solvent tends to permeate from the contact hole into theinterface between the resin and the underlying transparent conductivefilm, causing the resin film to peel from the transparent conductivefilm. In order to overcome this trouble, according to the active matrixsubstrate of this example, the metal nitride layer 41 made of TaN, AlN,and the like which have good adhesion with the resin is formed on thetransparent conductive film under the contact hole. Accordingly, thepeeling of the resin film and other troubles in the adhesion can beprevented.

Any material can be used for the metal nitride layer 41 as long as ithas good adhesion with the resin constituting the interlayer insulatingfilm 38, ITO and the like constituting the transparent conductive film37 a′, and metal such as Ta and Al. Such a material should also beelectrically conductive to electrically connect the transparentconductive film 37 a′ and the pixel electrode 21.

EXAMPLE 4

FIG. 6 is a partial sectional view of an active matrix substrate of atransmission type liquid crystal display device of Example 4 accordingto the present invention. This active matrix substrate has no channelprotection layer 35 as used in Example 2. As a result, the fabricatingprocess can be simplified and the material cost can be greatly reduced.

EXAMPLE 5

In Example 5, a method for driving the transmission type liquid crystaldisplay device according to the present invention will be described.

In the transmission type liquid crystal display device according to thepresent invention, each pixel electrode overlaps the corresponding linesvia the interlayer insulating film. If the pixel electrode does notoverlap the corresponding lines but gaps are formed therebetween,regions where no electric field is applied are formed in the liquidcrystal layer. This trouble can be avoided by overlapping the pixelelectrode with the lines. The electric field also is not applied to theregions of the liquid crystal layer corresponding to the boundaries ofthe adjacent pixel electrodes. However, light leakage which may occur atthese regions can be blocked by the existence of lines. This eliminatesthe necessity of forming a black mask on a counter substrate inconsideration of an error at the lamination of the active matrixsubstrate and the counter substrate. This improves the aperture ratio.Also, since the electric field generated at the lines can be shielded,disturbances in the orientation of the liquid crystal molecules can beminimized.

The overlap width should be set in consideration of a variation in theactual fabrication process. For example, it is preferably about 1.0 μmor more.

Crosstalk occurs due to the capacitance between the pixel electrode andthe source line when the pixel electrode overlaps the source line asdescribed above. This lowers the display quality of the resultanttransmission type liquid crystal display device. In particular, in aliquid crystal panel used for a notebook type personal computer wherepixels are arranged in a vertical stripe shape, the display is greatlyinfluenced by the capacitance between the pixel electrode and the sourceline. This is considered to be due to the following reasons: (1) Thecapacitance between the pixel electrode and the source line isrelatively large since, in the vertical stripe arrangement, the shape ofthe pixel electrode is rectangular having the side along the source lineas the major side; (2) Since the display color is different betweenadjacent pixels, there is little correlation between signals transmittedon the adjacent source lines. Thus, the influence of the capacitancecannot be cancelled between the adjacent source lines.

According to the transmission type liquid crystal display device of thepresent invention, the interlayer insulating film which is composed ofan organic thin film, i.e., a fluoric resin has an extremely smalldielectric constant and can be easily thickened. Therefore, thecapacitances between the pixel electrodes and the lines can be reduced,as described above. In addition to this feature, according to the methodfor driving the transmission type liquid crystal display device of thisexample, the influence of the capacitance between the pixel electrodeand the source line can be reduced to minimize vertical crosstalk whichoccurs in notebook type personal computers.

The method of this example includes driving the transmission type liquidcrystal display device by inverting the polarity of the data signal forevery horizontal scanning period (hereinafter, this method is referredto as “1H inversion driving”). Better display than that by the 1Hinversion driving method can be obtained when the transmission typeliquid crystal display device is driven by shifting the phase of thesource signal (i.e., data signal) from that of the gate signal, forexample, by about ½ phase of the gate signal. However, in this example,the 1H inversion will be described.

FIG. 7 shows the influences of the capacitance between the pixelelectrode and the source line upon the charging rate of the pixel in thecases of the 1H inversion driving and a driving method where thepolarity of the data signal is inverted every field (hereinafter, thismethod is referred to as “field inversion driving”). FIGS. 8A and 8Bshow the waveforms obtained by the 1H inversion driving and the fieldinversion driving, respectively.

In FIG. 7, the Y axis represents the charging rate difference whichindicates the ratio of the effective value of the voltage applied to theliquid crystal layer in the gray scale display portion when the grayscale is uniformly displayed to that when a black window pattern isdisplayed in the gray scale display at a vertical occupation of 33%. TheX axis represents the capacitance ratio which is proportional to thevariation in the voltage of the pixel electrode caused by thecapacitance between the pixel electrode and the source line, which isrepresented by expression (1) below:

Capacitance ratio=C _(sd)/(C _(sd) +C _(ls) +C _(s))  (1)

wherein C_(sd) denotes the capacitance value between the pixel electrodeand the source line, C_(ls) denotes the capacitance value of the liquidcrystal portion corresponding to each pixel at the gray scale display,and C_(s) denotes the capacitance value of the storage capacitor of eachpixel. The gray scale display refers to the display obtained when thetransmittance is 50%.

As is observed from FIG. 7, in the 1H inversion driving of this example,the influence of the capacitance between the pixel electrode and thesource line upon the effective voltage actually applied to the liquidcrystal layer can be reduced to one-fifth to one-tenth of that obtainedin the field inversion driving when the capacitance value is the same.This is because, in the 1H inversion driving, the polarity of the datasignal is inverted at intervals sufficiently shorter than the period ofone field during one field. This results in cancelling the influences ofthe positive signal and the negative signal on the display with eachother.

A display test was conducted using a VGA panel with a diagonal of 26 cm.From this test, it was observed that crosstalk was eminent when thecharging rate difference was 0.6% or more, degrading the displayquality. This is shown by the dotted curve in FIG. 7. From the curve inFIG. 7, it is found that the capacitance ratio should be 10% or less inorder to obtain the charging rate difference of 0.6% or less.

FIG. 9 shows the relationships between the overlap amount between thepixel electrode and the source line and the capacitance between thepixel electrode and the source line when the thickness of the interlayerinsulating film is used as a parameter. The VGA panel with a diagonal of26 cm was also used in this test. In the test, the fluoric resin(dielectric constant: approximately 2) used in Example 1 was used as theinterlayer insulating film. In consideration of the processingprecision, the overlap width between the pixel electrode and the sourceline should be at least 1 μm. From FIGS. 7 and 9, it is found that thethickness of the interlayer insulating film should be 1.2 μm or more,preferably 1.5 μm or more in consideration of variation which may occurin a mass-production process, to satisfy the overlap width of 1 μm andthe charging rate difference of 0.6% or less.

Thus, according to the 1H inversion driving of this example, a gooddisplay free from vertical crosstalk can be obtained without invertingthe polarity of the signal on the adjacent source lines (source lineinversion driving) when the pixel electrode overlaps the source line.This method is therefore applicable to notebook type personal computers.

It has also been found that a dot inversion driving has similar effectsto those obtained by the 1H inversion driving. The dot inversion drivingis a driving method where signals of the opposite polarities are inputinto pixel electrodes adjacent each other in the transverse directionand also the polarity is inverted every horizontal scanning period. Asource line inversion driving is also effective when the capacitanceratio is sufficiently low as in the above case. Further, even in thecolor display operation where adjacent signals are not highly correlatedwith each other, colored crosstalk may be suppressed, since thecapacitance between the pixel electrode and the source line issufficiently reduced according to the present invention.

EXAMPLE 6

In Example 6, another method for driving the transmission type liquidcrystal display device according to the present invention will bedescribed. In this method, the polarity of the voltage applied to theliquid crystal layer is inverted for every horizontal scanning period,and simultaneously the signal applied to the counter electrode is drivenby alternate current in synchronization with the inversion of thepolarity of the source signal. This AC driving of the counter electrodecan minimize the amplitude of the source signal.

FIG. 7 described in Example 5 also shows the curve obtained when thecounter electrode is AC driven with an amplitude of 5 V. From FIG. 7, itis observed that, since the 1H inversion driving is employed, thecharging rate difference is sufficiently small compared with the case ofthe field inversion driving, though it is larger by about 10 percentthan that obtained in Example 5 due to the AC driving of the counterelectrode in this example. As a result, a good display without verticalcrosstalk can be realized in the driving method of this example, as inthe previous example.

Although the transmission type liquid crystal display device ofC_(s)-on-Common type was used in the above example, the C_(s)-on-Gatetype can also be used.

EXAMPLE 7

FIG. 10 is a plan view of a one-pixel portion of an active matrixsubstrate of the transmission type liquid crystal display device ofExample 7.

In the transmission type liquid crystal display device of this example,each flat pixel electrode overlaps corresponding lines to improve theaperture ratio of the liquid crystal display, minimize disturbances inthe orientation of the liquid crystal molecules, and simplify thefabrication process. Also, the influence of the capacitances between thepixel electrode and the lines appearing on the display, such ascrosstalk, is minimized thereby achieving a good display.

Referring to FIG. 10, the active matrix substrate includes a pluralityof pixel electrodes 51 arranged in a matrix. Gate lines 52 and sourcelines 53 run along the peripheries of the pixel electrodes 51 to crosseach other. The peripheries of each pixel electrode 51 overlap the gatelines 52 and the source lines 53. A TFT 54 as a switching elementconnected to the corresponding pixel electrode 51 is formed at acrossing of the gate line 52 and the source line 53. A gate electrode ofthe TFT 54 is connected to the gate line 52 so that a gate signal isinput into the gate electrode to control the driving of the TFT 54. Asource electrode of the TFT 54 is connected to the source line 53 sothat a data signal can be input into the source electrode. A drainelectrode of the TFT 54 is connected to the corresponding pixelelectrode 51 via a connecting electrode 55 and a contact hole 56. Thedrain electrode is also connected to an electrode of a storagecapacitor, a storage capacitor electrode 55 a, via the connectingelectrode 55. The other electrode of the storage capacitor, a storagecapacitor counter electrode 57, is connected to a common line.

FIG. 11 is a sectional view of the active matrix substrate taken alongline C-C′ of FIG. 10.

Referring to FIG. 11, a gate electrode 62 connected to the gate line 52shown in FIG. 10 is formed on a transparent insulating substrate 61. Agate insulating film 63 is formed covering the gate electrode 62. Asemiconductor layer 64 is formed on the gate insulating film 63 so as tooverlap the gate electrode 62 via the gate insulating film 63, and achannel protection layer 65 is formed on the center of the semiconductorlayer 64. n⁺-Si layers as a source electrode 66 a and a drain electrode66 b are formed covering the end portions of the channel protectionlayer 65 and portions of the semiconductor layer 64, so that they areseparated from each other at the top of the channel protection layer 65.A transparent conductive film 67 a and a metal layer 67 b which are tobe the double-layer source line 53 shown in FIG. 10 are formed tooverlap the source electrode 66 a as one of the n⁺-Si layers. Atransparent conductive film 67 a′ and a metal layer 67 b′ are formed tooverlap the drain electrode 66 b as the other n⁺-Si layer. Thetransparent conductive film 67 a′ extends to connect the drain electrode66 b and the pixel electrode 51 and also serves as the connectingelectrode 55 which is connected to the storage capacitor electrode 55 a.An interlayer insulating film 68 is formed covering the TFT 54, the gateline 52, the source line 53, and the connecting electrode 55. Theinterlayer insulating film 68 is made of a fluoric resin.

A transparent conductive film is formed on the interlayer insulatingfilm 68 to constitute the pixel electrode 51. The pixel electrode 51 isconnected to the drain electrode 66 b of the TFT 54 via the contact hole56 formed through the interlayer insulating film 68 and the transparentconductive film 67 a′ which is the connecting electrode 55.

The active matrix substrate with the above structure is fabricated asfollows.

First, the gate electrode 62 made of Ta, Al, Mo, W, Cr, and the like,the gate insulating film 63 made of SiN_(x), SiO₂, Ta₂O₅, and the like,the semiconductor layer (intrinsic-Si) 64, the channel protection layer65 made of SiN_(x), Ta₂O₅, and the like, the n⁺-Si layers as the sourceelectrode 66 a and the drain electrode 66 b are sequentially formed inthis order on the transparent insulating substrate 61 such as a glasssubstrate.

Thereafter, the transparent conductive films 67 a and 67 a′ and themetal layers 67 b and 67 b′ made of Ta, Al, MoW, Cr, and the likeconstituting the source line 53 and the connecting electrode 55 aresequentially formed by sputtering and are patterned into a predeterminedshape. In this example, as in the previous examples, the source line 53is of the double-layer structure composed of the transparent conductivefilm 67 a made of ITO and the metal film 67 b. With this structure, ifpart of the metal layer 67 b is defective, the source line 53 can remainelectrically conductive through the transparent conductive film 67 a, sothat the occurrences of disconnection of the source line 53 can bereduced.

A fluoric resin is applied to the resultant structure to a thickness of2 μm, for example, by spin coating to form the interlayer insulatingfilm 68. The contact holes 56 through the interlayer insulating film 68are formed by a photolithography method.

Subsequently, a transparent conductive film is formed over theinterlayer insulating film 68 and the contact holes 56 by sputtering andis patterned to form the pixel electrodes 51. Thus, each pixel electrode51 is connected to the transparent conductive film 67 a′ which is incontact with the drain electrode 66 b of the TFT 54 via the contact hole56 formed through the interlayer insulating film 68. In this way, theactive matrix substrate of this example is fabricated.

Thereafter, ITO is deposited on the fluoric resin to a thickness of 50to 150 nm by sputtering and is patterned to form the pixel electrodes51. The ITO film as each pixel electrode 51 having a thickness of 50 nmor more effectively prevents an agent (e.g., dimethyl sulfoxide) used asa removing solution from permeating from gaps of the surface of the ITOfilm into the resin and the resin from expanding due to the permeationof the agent. The active matrix substrate of Example 7 is thusfabricated.

Thus, in this example, as in the previous examples, with the existenceof the interlayer insulating film 68, all the portions of the displaypanel other than the source and gate line portions can be used as pixelaperture portions. The resultant liquid crystal display device is brightwith high transmittance and a large aperture ratio.

Moreover, with the existence of the interlayer insulating film 68, thepixel electrodes can be made flat without being influenced by stepsformed by the underlying lines and switching elements. This prevents theoccurrence of disconnection conventionally found at the steps on thedrain sides of the pixel electrodes, and thereby reduces the number ofdefective pixels. Disturbances in the orientation of liquid crystalmolecules caused by the steps can also be prevented. Furthermore, sincethe source lines 53 and the pixel electrodes 51 are isolated from eachother with the interlayer insulating film 68 therebetween, the number ofdefective pixels conventionally caused by the electrical leakage betweenthe source lines 53 and the pixel electrodes 51 can be reduced.

EXAMPLE 8

In Example 8, the method for improving the adhesion between theinterlayer insulating film 68 and the underlying films described inExample 7 shown in FIGS. 10 and 11 will be described.

The adhesion of the fluoric resin as the interlayer insulating film 68with the underlying films may be inferior depending on the materials ofthe underlying films. In such a case, according to the method of thisexample, the surfaces of the underlying films, i.e., the gate insulatingfilm 63, the channel protection film 65, the source electrode 66 a, thedrain electrode 66 b, the transparent conductive films 67 a and 67 a′,and the metal films 67 b and 67 b′ are exposed to ultraviolet light froman M-type mercury lamp (860 W) in an oxygen atmosphere before theapplication of the fluoric resin, so as to roughen the surfaces. Theinterlayer insulating film 68 made of the fluoric resin is then formedon the roughened surfaces of the underlying films. The subsequent stepsare the same as those described in Example 7. By this method, theadhesion between the fluoric resin and the surface-roughened underlyingfilms improves. This overcomes the conventional problem of the filmpeeling at the interface between the interlayer insulating film 68 madeof the fluoric resin and the underlying films. This condition resultswhen an agent such as a mixture of hydrochloric acid and iron chloridefor etching ITO, permeates into the interface.

Thus, by irradiating the substrate surface before the formation of theinterlayer insulating film 68 with ultraviolet light, the adhesionbetween the interlayer insulating film 68 and the underlying filmsimproves. The resultant device can be stable despite further processingduring the fabrication process.

An alternative method for improving the adhesion according to thepresent invention is to treat the surface to be coated with the resinwith a silane coupling agent before the coating with the resin. As thesilane coupling agent, hexamethyl disilazane, dimethyl diethoxy silane,n-buthyl trimethoxy silane, and the like are especially effective inimprovement of the adhesion. For example, in the case of adhesion withthe silicon nitride film, it has been found that the adhesion strengthof the treated surface improves by about 10% compared with that of thesurface not treated with the silane coupling agent. The problem that thepattern of the resin is damaged due to an internal stress generated bythe crosslinking of the resin, which sometimes occurs if the surface isnot so treated, is prevented by this treatment with the silane couplingagent.

The silane coupling agent may be blended in the resin before theapplication of the resin, instead of applying the agent to theunderlaying layer before the application of the resin. The same adhesioneffect can be obtained by this method. Specifically, when 1 wt % ofdimethyl diethoxy silane was added to the fluoric resin, the adhesionstrength of the resin with the silicon nitride film (i.e., a underlaying layer) improved by 70%.

EXAMPLE 9

In Example 9, the method for improving the adhesion between theinterlayer insulating film 68 and the pixel electrode material formedthereon described in Example 7 and shown in FIGS. 10 and 11 will bedescribed.

After the formation of the interlayer insulating film 68 made of thefluoric resin in Example 7, the surface portion of the interlayerinsulating film 68 with a thickness of 100 to 500 nm is ashed in anoxygen plasma atmosphere using a dry etching apparatus. Morespecifically, the surface of the fluoric resin is ashed in the oxygenplasma atmosphere using a parallel plane type plasma etching apparatusunder the conditions of a RF power of about 1.2 KW, a pressure of about800 m Torr, an oxygen flow rate of about 300 sccm, a temperature of 70°C., and a RF applying time of about 120 seconds. By this process, waterand carbon dioxide are released from the surface of the fluoric resin byoxidation decomposition, and thus the surface is roughened.

Thereafter, ITO is deposited on the roughened fluoric resin to athickness of about 50 to about 150 nm by sputtering and patterned toform the pixel electrodes 51. The active matrix substrate is thusfabricated.

By this ashing, the adhesion between the pixel electrodes 51 and theunderlying roughened interlayer insulating film 68 made of the fluoricresin greatly improves. No delamination at the interface thereof wascaused by an application of ultrasound for cleaning the substrate. Theabove effect was not obtained when the thickness of the ashed surfaceportion of the fluoric resin was less than 100 nm. When it exceeds 500nm, the decrease in the thickness of the fluoric resin is so large thatthe variation in the thickness of the resultant fluoric resin increases,causing display troubles. The improvement in the adhesion is obtained byusing any type of the dry etching apparatus including a barrel type anda RIE type.

Thus, by ashing the surface portion of the interlayer insulating film 68in the oxygen plasma atmosphere before the formation of the pixelelectrodes, the adhesion between the interlayer insulating film 68 andthe pixel electrode material improves. The resultant device can bestable against further processing during the fabrication process. Inaddition, the ashing is also effective in removing residues from thecontact holes. This reduces the occurrence of disconnection in thecontact holes.

EXAMPLE 10

FIG. 14 is a plane view of an active matrix substrate of thetransmission type liquid crystal display device of Example 10 accordingto the present invention. FIG. 15 is a sectional view taken along lineD-D′ of FIG. 14. Components having like functions and effects aredenoted by the same reference numerals as those in FIGS. 1 and 2, andthe description thereof is omitted.

In the active matrix substrate of this example, the connections betweeneach TFT 24 and the corresponding pixel electrode 21 and between eachstorage capacitor electrode 25 a and the corresponding pixel electrode21 are effected via separate contact holes 26 a and 26 b, respectively.Also, in this example, each source line 23 is composed of a single metallayer, though it may be of a multi-layer structure. The storagecapacitor electrodes 25 a are formed of the same material as that of thesource lines 23 in the same step as in the previous examples. The twocontact holes 26 a and 26 b are formed above a metal electrode 23 bconnected to the drain electrode 36 b of the TFT and above the storagecapacitor electrode 25 a, respectively. That is, these contact holes 26a and 26 b are formed above the metal electrodes having a light-shadingproperty.

The transmission type liquid crystal display device with the abovestructure is advantageous in the following points.

When the thickness of the interlayer insulating film 38 is as large as 3μm, for example, which is well comparable with the typical thickness ofa liquid crystal layer (a cell thickness) of 4.5 μm , light leakagetends to occur around the contact holes 26 a and 26 b due todisturbances in the orientation of the liquid crystal molecules. If thecontact holes 26 a and 26 b are formed in the aperture portions of thetransmission type liquid crystal display device, the contrast ratio islowered due to the light leakage. In contrast, the active matrixsubstrate of this example is free from this trouble because the storagecapacitor electrode 25 a blocks the light from around the contact holes26 b and the metal electrode 23 b blocks the light from around thecontact holes 26 a. The aperture ratio can be further increased byforming the storage capacitor counter electrodes 27 so that they do notextend from the storage capacitance electrode 25 a. Though theC_(s)-on-Common type was used in this example, the C_(s)-on-Gate typecan also be used.

Thus, in Examples 1 to 10 above, each pixel electrode overlaps thecorresponding lines to improve the aperture ratio of the liquid crystaldisplay, to minimize disturbances in the orientation of the liquidcrystal molecules, and to simplify the fabrication process. Also, theinfluence of the capacitances between the pixel electrode and the linesappearing on the display, such as crosstalk, is minimized to achieve agood display. In addition to these features, a wide viewing angle can beobtained.

The wide viewing angle can be obtained due to the following reasons: (1)The orientation of the liquid crystal molecules is not disturbed sincethe surfaces of the pixel electrodes are flat; (2) No disclination lineis generated due to the electric field generated at the lines; (3)oblique light from the backlight can be effectively used by having theinterlayer insulating film 38 as thick as several micrometers while thedistance between adjacent aperture portions is in the range of severalmicrons to ten and several microns; and (4) The contrast is large (1:300or more for a 10.4-inch SVGA). As a result, the retardation value, i.e.,the refractive index anisotropy of liquid crystal (Δn)×cell thickness(d), can be reduced. This reduction of the retardation is obtainedmainly by reducing the cell thickness according to the presentinvention. In general, as the value of Δn×d decreases, the viewing angleincreases but the contrast decreases. According to the presentinvention, however, the size of the pixel electrodes is made large byeliminating the margins conventionally provided between the pixelelectrodes and the corresponding lines. For example, for a 10.4″ VGA,the aperture ratio increased by about 20 points from 65% to 86%, and thebrightness also increased by more than 1.5 times. For a 12.1″ XGA,similarly, the aperture ratio greatly increased from 55% to 80%. Thereason is as follows. In the conventional structure, when the sourceline width is 6 μm, the gap between the source line and the pixelelectrode is 3 μm, and the attachment margin is 5 μm, for example, thedistance between adjacent aperture portion is required to be 22 μm ormore. In contrast, according to the present invention where each pixelelectrode overlaps the corresponding source lines, the distance betweenadjacent aperture portions can be 6 μm which is the source line width.Thus, the ratio of the portion which does not constitute the apertureportion to the entire area can be greatly reduced.

Examples 2 and 3 described the transmission type liquid crystal displaydevice where one electrode of the storage capacitor (a storage capacitorelectrode) is connected to the counter electrode via the storagecapacitor common line. The same effects obtained by the above structurecan also be obtained by using the gate line 22 of the adjacent pixel asthe storage capacitor electrode. FIGS. 12 and 13 show the latterstructure. This type of liquid crystal display device is called aC_(s)-on-gate type, where each pixel electrode 21 overlaps theimmediately before or next gate line 22 to form a storage capacitorC_(s). In this case, the pixel electrode 21 preferably overlaps a largerportion of the immediately before or next gate line while it overlaps asmaller portion of the corresponding gate line.

In Examples 1 to 10, the fluoric resin with high transparency is appliedby spin coating and patterned to form the interlayer insulating film,and the contact holes are formed through the interlayer insulating film.The application of the fluoric resin can also be conducted by methodsother than the spin coating, such as roll coating and slot coating. Theeffects of the present invention can also be obtained by these methods.Roll coating is a method where a substrate is allowed to pass throughbetween a roll with an uneven surface and a belt with the surface of thesubstrate to be subjected to the coating facing the roll. The thicknessof the resultant coating is determined by the degree of the unevenness.The slot coating is a method where a substrate is allowed to pass underan ejection slot. The thickness of the resultant coating is determinedby the width of the ejection slot.

In Example 8, among the i line (wavelength: 365 nm), the h line(wavelength: 405 nm), and the g line (wavelength: 436 nm) generally usedfor the light exposure process, the i line having the shortestwavelength is used. This shortens the light irradiation time, and ishighly effective in roughening the surface in Example 8.

Thus, according to the present invention, with the existence of theinterlayer insulating film, each pixel electrode can be formed tooverlap the corresponding lines. This improves the aperture ratio andminimizes disturbances in the orientation of the liquid crystalmolecules. Since the interlayer insulating film is composed of a fluoricresin thin film, the dielectric constant thereof is smaller and thethickness thereof can be easily larger, compared with an inorganic thinfilm. Thus, the capacitances between the pixel electrode and the linescan be reduced. As a result, vertical crosstalk caused by thecapacitance between the pixel electrode and the source line can bereduced, and the feedthrough of the write voltage to the pixels causedby the capacitance between the pixel electrode and the gate line, aswell as the variation in the fabrication process, can be reduced.

In the formation of the interlayer insulating film, the fluoric resin isapplied to the substrate by a coating method such as spin coating, rollcoating or slot coating, and then cured. Thereafter, the resultantsubstrate is shielded with a resist, exposed to light using the resistas a mask and subjected to a dry etching process. This makes it possibleto fabricate a transmission type liquid crystal display device having ahigh aperture ratio at reduced production cost compared with the CVDmethod and with better processing precision compared with other resins.

The connecting electrode for connecting the drain electrode of the TFTand the pixel electrode is formed using the transparent conductive film.This further improves the aperture ratio. The transparent conductivefilm can be formed simultaneously with the source line which is of adouble-layer structure including the transparent conductive film. Withthe double-layer structure, disconnection at the source line can beprevented.

Each contact hole is formed through the interlayer insulating film abovethe storage capacitor common line or the gate line (i.e., scanningline). This improves the contrast ratio because light leakage which maybe generated due to a disturbance in the orientation of the liquidcrystal can be blocked by the storage capacitor portion. In other words,light leakage is generated in the light-shading portions, if generated,not in the aperture portions.

The metal nitride layer may be formed under each contact hole formedthrough the interlayer insulating film. This improves the adhesionbetween the interlayer insulating film and the underlying film. Thus,the resultant liquid crystal display device is stable against furtherprocessing in the production process.

Each pixel electrode ma y overlap the corresponding source line by 1 μmor more. With this overlap, the aperture ratio can be maximized. Also,the processing precision of each pixel electrode with respect to thecorresponding lines is not necessarily required to be high. This isbecause, even if the processing precision is low, light leakage can bewell blocked by the overlapping lines as long as the pixel electrodeoverlaps the lines.

By having the thickness of the interlayer insulating film be 1.2 μm(preferably, 1.5 μm or more), the capacitance between each pixelelectrode and the corresponding source line can be sufficiently small.This reduces the time constant even though the pixel electrode overlapsthe source line by 1 μm or more. As a result, the influence of thecapacitance appearing on the display, such as crosstalk, can be reduced,and thus a good display can be provided.

The vertical crosstalk is further reduced by decreasing the capacitanceratio represented by expression (1) above to 10% or less, since thecapacitance between the pixel electrode and the source line issufficiently reduced.

The polarity of the data signal supplied from the source line may beinverted every gate line. This further reduces the influence of thecapacitance between each pixel electrode and the corresponding sourceline appearing on the display, such as vertical crosstalk.

The effects of the present invention can also be obtained for the activematrix substrate where the pixel electrodes are arranged in a verticalstripe shape and each pixel electrode is of a rectangular shape with theside thereof parallel to the source line being longer than the sidethereof parallel to the gate line. This makes it possible to obtain alarge-scale liquid crystal display device with a high aperture ratiofree from vertical crosstalk for notebook type personal computers andthe like.

Each storage capacitor is formed using an insulating film which isextremely thinner than the interlayer insulating film. The resultantstorage capacitor can have a large capacitance while the area thereof issmall. This improves the aperture ratio. Since the storage capacitorelectrodes are formed simultaneously with the source lines (i.e., signallines), an increase in the number of process steps can be avoided.

When the source lines are composed of light-shading conductive films,the contact hole portions can be blocked from light. This concealsdisturbances in the orientation of the liquid crystals occurring atthese portions, improving the display quality. This also improves theaperture ratio.

Since the interlayer insulating film according to the present inventionis colorless, the amount of light from a backlight can be reduced,saving power consumption, or the brightness can be increased if theamount of light from the backlight is not reduced.

Since the interlayer insulating film according to the present inventionis comparatively thick and can be made flat, conventional troublescaused by steps formed by the underlying lines and the like, such asdisconnection on the drain side of the pixel electrode, are overcome.Disturbances in the orientation of the liquid crystal is also prevented.The pixel electrodes and the lines are isolated by the interlayerinsulating film formed therebetween. This greatly reduces the number ofdefective pixels due to electrical leakage between the pixel electrodesand the lines, thereby increasing production yield and reducingproduction cost.

The surface of the substrate before the formation of the interlayerinsulating film may be irradiated with ultraviolet light. This improvesthe adhesion between the interlayer insulating film and the underlyingfilm. Thus, the resultant liquid crystal display device can be stableagainst further processing in the production process.

The surface of the interlayer insulating film may be ashed in an oxygenplasma atmosphere before the formation of the film of pixel electrodematerial. This improves the adhesion of the interlayer insulating filmand the film of the pixel electrode material formed thereon. Thus, theresultant liquid crystal display device can be stable against furtherprocessing in the production process.

The pixel electrodes with a thickness of 50 nm or more can effectivelyprevent an agent used as a removing solution from permeating from gapsof the film surface into the resin and the resin from expanding due tothe permeation of the agent.

The light irradiation time can be shortened by using the i line(wavelength: 365 nm) having higher energy than visible light.

As the aperture ratio of the display improves, the brightness thereofalso improves. Accordingly, the viewing angle can be widened by reducingthe retardation without degrading the contrast. This makes it possibleto obtain a significantly wide viewing angle.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A transmission type liquid crystal display devicecomprising: gate lines; source lines; and switching elements eacharranged near a crossing of each gate line and each source line, a gateelectrode of each switching element being connected to the gate line, asource electrode of the switching element being connected to the sourceline, and a drain electrode of the switching element being connected toa pixel electrode for applying a voltage to a liquid crystal layer,wherein an interlayer insulating film formed of a fluoric resin withhigh transparency is provided above the switching element, the gateline, and the source line, the pixel electrode formed of a transparentconductive film is provided on the interlayer insulating film; andwherein a connecting electrode is formed of a transparent conductivefilm.
 2. A transmission type liquid crystal display device comprising:gate lines; source lines; and switching elements each arranged near acrossing of each gate line and each source line, a gate electrode ofeach switching element being connected to the gate lines a sourceelectrode of the switching element being connected to the source line,and a drain electrode of the switching element being connected to apixel electrode for applying a voltage to a liquid crystal layer,wherein an interlayer insulating film formed of a fluoric resin withhigh transparency is provided above the switching element the gate line,and the source line, the pixel electrode formed of a transparentconductive film is provided on the interlayer insulating film, andfurther comprising a connecting electrode for connecting the pixelelectrode and the drain electrode, wherein the interlayer insulatingfilm is provided above the switching element, the gate line, the sourceline, and the connecting electrode, the pixel electrode is formed on theinterlayer insulating film so as to overlap at least the gate line orthe source line at least partially, and the connecting electrode and thepixel electrode are connected with each other via a first contact holeformed through the interlayer insulating film, wherein a metal nitridelayer is formed below the first contact hole to connect the connectingelectrode and the pixel electrode.
 3. A transmission type liquid crystaldisplay device comprising: gate lines; source lines; and switchingelements each arranged near a crossing of each gate line and each sourceline; an interlayer insulating film formed of a fluoric resin with hightransparency provided above the switching element, the gate line, andthe source line; and a pixel electrode formed of a transparentconductive film provided on the interlayer insulating film, a gateelectrode of each switching element being connected to the gate line, asource electrode of the switching element being connected to the sourceline, and a drain electrode of the switching element being connected toa pixel electrode for applying a voltage to a liquid crystal layer,wherein the transmission type liquid crystal display device furthercomprises a driving circuit for supplying to the source line a datasignal of which polarity is inverted for every horizontal scanningperiod, and the data signal is supplied to the pixel electrode via theswitching element, the data signal having a phase difference with a gatesignal.
 4. A transmission type liquid crystal display device comprising:gate lines; source lines; and switching elements each arranged near acrossing of each gate line and each source line, a gate electrode ofeach switching element being connected to the gate line, a sourceelectrode of the switching element being connected to the source line,and a drain electrode of the switching element being connected to apixel electrode for applying a voltage to a liquid crystal layer,wherein a interlayer insulating film formed of a fluoric resin having alight transmittance of 90% or more for light with a wavelength in therange of about 400nm to 800 nm is provided above the switching element,the gate line, and the source line, the pixel electrode formed of atransparent conductive film is provided on the interlayer insulatingfilm, wherein the interlayer insulating film includes either a fluoricpolyimide or a fluoric acrylate resin.
 5. A transmission type liquidcrystal display device according to claim 4, wherein the interlayerinsulating film has a thickness of 1.2 μm or more.
 6. A transmissiontype liquid crystal display device according to claim 5, wherein thepixel electrode and at least one of the source line and the gate lineoverlap each other by about 1 μm or more in a line width direction.
 7. Atransmission type liquid crystal display device according to claim 4,further comprising a storage capacitor for holding a voltage applied tothe liquid crystal layer, wherein a capacitance ratio represented byexpression (1): Capacitance ratio=C _(sd)/(C _(sd) +C _(1s) +C_(s))  (1) is 0.1 or less, wherein C_(sd) denotes a capacitance valuebetween the pixel electrode and the source line, C_(1s) denotes acapacitance value of a liquid crystal portion corresponding to eachpixel in an intermediate display state, and C_(s) denotes a capacitancevalue of the storage capacitor of each pixel.
 8. A transmission typeliquid crystal display device according to claim 4, wherein the shape ofthe pixel electrode is rectangular with a side parallel to the gate linebeing shorter than a side parallel to the source line.
 9. A transmissiontype liquid crystal display device according to claim 4, furthercomprising a storage capacitor for holding a voltage applied to theliquid crystal layer, the storage capacitor including: a storagecapacitor electrode; a storage capacitor counter electrode; and aninsulating film therebetween; wherein the storage capacitor electrode isformed in the same layer as either the source line or the connectingelectrode.
 10. A transmission type liquid crystal display deviceaccording to claim 9, wherein the storage capacitor counter electrode isformed of a part of the gate line.
 11. A transmission type liquidcrystal display device according to claim 10, wherein the pixelelectrode and the storage capacitor electrode are connected via a secondcontact hole formed above the storage capacitor electrode.
 12. Atransmission type liquid crystal display device according to claim 9,wherein a second contact hole is formed above either the storagecapacitor counter electrode or the gate line.
 13. A transmission typeliquid crystal display device according to claim 5, further comprising aconnecting electrode for connecting the pixel electrode and the drainelectrode, wherein the interlayer insulating film is provided above theswitching element, the gate line, the source line, and the connectingelectrode, the pixel electrode is formed on the interlayer insulatingfilm so as to overlap at least the gate line or the source line at leastpartially, and the connecting electrode and the pixel electrode areconnected with each other via a first contact hole formed through theinterlayer insulating film.
 14. A transmission type liquid crystaldisplay device according to claim 13, further comprising a storagecapacitor for maintaining a voltage applied to the liquid crystal layer,wherein the first contact hole is formed through the interlayerinsulating film above either an electrode of the storage capacitor orthe gate line.
 15. A transmission type liquid crystal display deviceaccording to claim 4, wherein the interlayer insulating film comprisesfluoric polyimide.
 16. A transmission type liquid crystal display deviceaccording to claim 15, wherein the connecting electrode is formed of atransparent conductive film.
 17. A transmission type liquid crystaldisplay device according to claim 15, wherein a metal nitride layer isformed below the contact hole to connect the connecting electrode andthe pixel electrode.
 18. A transmission type liquid crystal displaydevice according to claim 4, wherein the interlayer insulating filmcomprises fluoric acrylate resin.
 19. A transmission type liquid crystaldisplay device according to claim 18, wherein the fluoric acrylate resinis photosensitive.
 20. A transmission type liquid crystal display deviceaccording to claim 15, wherein the fluoric polyimide contains both acidanhydride and diamine each having hexafluoropropylene.
 21. Atransmission type liquid crystal display device according to claim 4,wherein the thickness of the interlayer insulating film is 1.2 μm.
 22. Aliquid crystal display device comprising: gate lines; source lines; andswitching elements each arranged near a crossing of each gate line andeach source line, a gate electrode of each switching element beingconnected to the gate line, a source electrode of the switching elementbeing connected to the source line, and a drain electrode of theswitching element being connected to a pixel electrode for applying avoltage to a liquid crystal layer, wherein an interlayer insulating filmformed of a fluoric resin is provided above the switching element, thegate line, and the source line, the pixel electrode formed of aconductive film is provided on the interlayer insulating film, andfurther comprising a connecting electrode for connecting the pixelelectrode and the drain electrode, wherein the interlayer insulatingfilm is provided above the switching element, the gate line, the sourceline, and the connecting electrode, the pixel electrode is formed on theinterlayer insulating film so as to overlap at least the gate line orthe source line at least partially, and the connecting electrode and thepixel electrode are connected with each other via a first contact holeformed through the interlayer insulating film, wherein a metal nitridelayer is formed below the first contact hole to connect the connectingelectrode and the pixel electrode.
 23. A liquid crystal display devicecomprising: gate lines; source lines; and switching elements eacharranged near a crossing of each gate line and each source line; aninterlayer insulating film formed of a fluoric resin provided above theswitching element, the gate line, and the source line; and a pixelelectrode formed of a conductive film provided on the interlayerinsulating film, a gate electrode of each switching element beingconnected to the gate line, a source electrode of the switching elementbeing connected to the source line, and a drain electrode of theswitching element being connected to a pixel electrode for applying avoltage to a liquid crystal layer, wherein the liquid crystal displaydevice further comprises a driving circuit for supplying to the sourceline a data signal of which polarity is inverted for every horizontalscanning period, and the data signal is supplied to the pixel electrodevia the switching element, the data signal having a phase differencewith a gate signal.